By Alicia Hartlieb and Martin Hartlieb, Viami International Inc.,
and Julio Quintero, Conticast Hormesa LLC.
Introduction
Traditionally, primary casthouses solidified the molten primary aluminum from the smelter’s potlines into pure ingot or sow, or they produced alloyed ingot, billet, or slab. Secondary casthouses served to convert scrap into secondary aluminum ingot, billet, or slab. Today, the global trend toward sustainability and building a circular economy has led to a focus on maximizing recycling in order to minimize carbon footprint. As a result, almost every casthouse has begun to integrate scrap into its production mix.
Secondary casthouses no longer solely produce higher impurity secondary alloys, but also high-quality ingot, billet, and slab with the highest possible recycled content. This trend is changing the way the aluminum industry recycles, the way it sorts and treats scrap, and the way modern secondary casthouses need to be designed with energy efficiency, optimum recovery, and metal quality at the focus. The industry is working to improve closed-loop recycling where possible, and where it’s not, then it is working to achieve the best possible integration of recycled content in aluminum products.
Beside the input material, the melting step is the most energy and carbon intensive process in a secondary casthouse. Selecting the right furnace technology for each scrap type and available energy is therefore vital for maximizing recoveries and minimizing energy usage and carbon footprint. Numerous R&D projects are under way to improve scrap sorting and treatment, energy efficiency and recovery, up-cycling scrap into high quality alloys, and to develop alloys that can be made from the highest possible (ideally post-consumer) recycling content.
Carbon Footprint of Recycled Aluminum
With the increasing interest in the environmental impact of aluminum production, casthouses are often faced with the task of calculating the carbon footprint of their products. This can be made according to different standards, the most common being the Greenhouse Gas Protocol Corporate Standard (with Scope 1 to 3 emissions being tracked). A large portion of the carbon footprint in any aluminum product originates from the raw materials used—even when it is scrap.
Post-consumer scrap is by definition always reset to zero carbon footprint at the end of its life cycle.1 For pre-consumer scrap (also known as process scrap), the carbon footprint may be calculated in various ways—one of which is the cut-off method, in which any scrap created during production is considered emission-free and the carbon footprint of all materials and scrap are allocated to the final product.1 Given that the scrap is considered carbon footprint free in this case, when this scrap is utilized elsewhere, no additional carbon footprint is added to that final product. An alternative reporting method is the co-product method, in which production waste carries the same carbon footprint as the primary product from which it was created.1
Certain guidelines, such as the Rocky Mountain Institute (RMI) guide, state that, when in doubt, casthouses should report according to the cut-off method and only use the co-product method if they believe the data on the carbon footprint of their scrap is precise enough.2 It is important to note that following this guideline may lead to accusations of greenwashing, due to the fact that it encourages a lower traceability of scrap and defaulting to the assumption of it being carbon-free. This raises the question of whether it would be more accurate to use published average numbers for scrap carbon footprint instead of assuming it to be zero in cases where the data is considered to be inaccurate or unreliable.
Secondary Aluminum Casthouses
Secondary casthouses make use of aluminum scrap by converting it to secondary aluminum ingot, billet, or slab (Figure 1). The first step to this is purchasing the collected scrap from a scrapyard, where initial sorting is often already done, as well as potentially purchasing primary aluminum if needed. The types of scrap chosen by a casthouse depend on the products they intend to make, as well as the availability and price of certain scrap types.
A critical component in a casthouse’s process is to reduce or minimize the number of impurities in their products, given that the scrap being used comes from different sources and likely has varying alloying elements and impurities. There are two methods that can be used to control impurities once the scrap is molten, namely downgrading and dilution. Downgrading involves creating high-alloyed recycled aluminum by combining low-alloyed scrap, whereas in dilution primary aluminum is added to the molten scrap so that the concentration of certain elements is reduced until the desired composition is reached. However, these methods are not always viable.4
Another potential solution is to improve the entire recycling process, where first the scrap receives various treatments before melting. One way to do this is through improved sorting, such as using technologies like laser-induced breakdown spectroscopy (LIBS) or vision and x-ray fluorescence systems to ensure stricter homogeneity within the scrap. Additional methods include breaking the scrap down into smaller pieces, using thermal treatment for the removal of moisture and scrap coatings (i.e., paint and contaminants), and/or mechanical treatment. While such process steps are an additional investment, they increase the quality of scrap and its recyclability.4
The next step is to ensure that the right scrap mix is melted for each product (sometimes combined with primary aluminum and alloying ingredients). After melting, degassing, fluxing, and/or refining can be performed, which entails adding chemicals to clean the molten aluminum and hinder the formation of oxide. These processes remove hydrogen, as well as metallic and non-metallic impurities, and reduce metal loss.4 Additionally, eutectic modification and grain refining are common casthouse functions.5
Once the desired alloy composition is attained, casting the molten aluminum into ingot, billet, or slab may take place. The casting process can be performed using open mold, direct chill, or other production methods. Finally, the billet or slab may receive homogenization or heat treatment before they are utilized elsewhere.5
As shown in Figure 1, the aluminum recycling process also requires energy (gas and/or electricity), alloying additives, fluxes for cleaning the melt, refractories, cooling water, and air and gases for the burners, such as argon, nitrogen, and oxygen. In addition, the plant requires post-casting processes, such as dross treatment.
Trends and Challenges for Secondary Casthouses
Better identification of scrap types and alloys through sensor-based sorting is quickly improving with new technologies in this field, such as LIBS, which allows a rapid chemical analysis through a short laser pulse to the surface of the metal. This technology detects a wide variety of elements present without the metal requiring any preparation, allowing a much more precise sorting of scrap.6 Alternate technologies to improve scrap sorting include using a vision system (patent US10710119 from UHV Technologies, Inc.), which is often used in combination with x-ray fluorescence. This sorting method usually involves the scrap being carried on a conveyor system, which then delivers it to the machine that identifies and classifies the various materials, according to which the scrap is then sorted.5
Once the scrap is sorted and segregated, cleaning is the next challenge. Delacquering is important both for metal quality as well as for environmental reasons. This can be done through mechanical pre-treatment (like shot blasting), chemical or biological treatments, or thermal treatment.
Besides optimizing the scrap mix through sorting (to minimize the need for primary aluminum and alloying elements required during casting), technologies to improve energy efficiency and recovery are the most important topics to minimize carbon footprint, and their usage is on the rise. To achieve this, all aspects of the process need to be optimized. Using the right technologies for the most energy intensive process step—melting—is vital and many innovations regarding burner technologies (e.g. regenerative burners), adaptive temperature control (dynamically adjusting the melting conditions to suit the composition of the scrap), optimized furnace layout/design and insulation, as well as recovery and reuse of exhaust gases are key. As an example of these types of technologies, Figure 2 shows a scrap delacquering system, and Figure 3 shows a twin chamber furnace with exhaust gas recirculation. Using the right technology for each scrap type is not only relevant for energy efficiency, but also important for maximizing metal yield and recovery. Optimizing energy efficiency and recovery not only increases sustainability and reduces carbon footprint, but it also helps the casthouse to become more economical and competitive.
Advanced melting technologies utilize selective melting techniques based on the type of aluminum scrap. This ensures optimal melting efficiency and recovery rates by tailoring the melting process to the specific characteristics of input materials. Table I shows examples of scrap types and achievable melting efficiencies when using the right technologies.
The overall trend within casthouses is to utilize and optimize environmentally friendlier procedures beyond the scrap sorting/treatment and melting step, which is apparent through the rise of patents on technologies, such as ultrasonic grain refining and degassing,5 which have been created for aluminum recycling.
Various new technologies under development are precisely targeting the removal of impurities from the melt. Another method involves the conversion of those impurities into alloying constituents or at least making them “harmless” for the alloy properties by adding elements like new grain refiners into the melt.
A secondary casthouse mainly melting scrap will unavoidably generate a lot more dross than a primary casthouse—typically anywhere from 3 to 10%. Minimizing dross and optimum treatment of it are therefore vital. New technologies for dross recycling/treatment include plasma treatment, which can significantly increase the recovery rate (compared to traditional rotary salt furnaces), as well as significantly reduce energy consumption and carbon footprint and avoid salt contamination and the formation of hazardous salt cakes.
Conclusion
Secondary aluminum casthouses are embracing a transformative journey towards sustainability, while at the same time producing much more sophisticated products that often replace some that were traditionally made only from primary aluminum. The demand from customers for high quality ingot, billet, or slab with the highest quality, highest amount of (ideally, post-consumer) recycled content, and lowest carbon footprint is driving this trend. Technologies across all casthouse processes are evolving and significantly improving their performance. These developments start with much improved scrap sorting/segregation and treatment and continue through to improved melting, alloying, and melt treatment technologies, and continues all the way to improved dross processing—all of which are able to improve metal yield and recovery, while minimizing emissions and waste generation.
References
- Hartlieb, Alicia, and Dr. Subodh Das, “The Carbon Footprint of Recycled or Secondary Aluminum” Light Metal Age, Vol. 81, No. 4, August 2023.
- Brown, Lynne, and Shane Tredup, “Supporting Extruders in Navigating the Environmental Landscape: AEC Develops a New Member Tool for EPDs,” Light Metal Age, Vol. 82, No. 3, April 2024.
- Wei, Wenjing, “Energy Consumption and Carbon Footprint of Secondary Aluminum Cast House,” Environmental Science, Engineering, 2012.
- Capuzzi, Stefan, and Giulio Timelli, “Preparation and Melting of Scrap in Aluminum Recycling: A Review,” Metals, Vol. 8, No. 4 April 2018.
- “International Patents: Aluminum Remelting and Casthouse Strategies,” Light Metal Age, August 18, 2020.
- “What is LIBS,” Applied Spectra.
Editor’s Note: This article first appeared in the August 2024 issue of Light Metal Age. To receive the current issue, please subscribe.
Alicia Hartlieb is a science student at Queen’s University (Kingston, ON) and has been working for Viami International Inc. since 2020, doing market/data research/analysis, content writing and editing, translations, etc. for various companies in the aluminum industry.
Martin Hartlieb is president of Viami International Inc. and has over 25 years of experience in the aluminum industry (and 23 years of those with structural castings). He is a consultant/ advisor for many companies across the value chain, including aluminum alloy producers, recyclers, technology providers, processors (especially foundries/die casters) and users/OEMs.
Julio Quintero is a business development manager at Hormesa. He leverages his engineering background and foundry expertise to provide technological solutions that optimize productivity and efficiency and support aluminum companies to develop projects and initiatives that achieve sustainability and net zero carbon emissions goals.